The recent miniaturization and the increased levels of integration of semiconductor integrated circuits have led to a demand for increasing in a resolution of semiconductor exposure apparatuses. A semiconductor exposure apparatus is hereinafter referred to simply as “exposure apparatus”. Accordingly, exposure light sources to emit light at shorter wavelengths have been under development. As the exposure light sources, gas laser apparatuses instead of conventional mercury lamps are typically used. The gas laser apparatuses for exposure include a KrF excimer laser apparatus that emits an ultraviolet laser beam at a wavelength of 248 nm and an ArF excimer laser apparatus that emits an ultraviolet laser beam at a wavelength of 193 nm.
As an advanced exposure technology, immersion exposure has been put into practical use. In the immersion exposure, a gap between an exposure lens and a wafer in an exposure apparatus is filled with a fluid such as water. The immersion exposure allows the refractive index of the gap to be changed and thus an apparent wavelength of the light from the exposure light source is shortened. The immersion exposure using an ArF excimer laser apparatus as an exposure light source allows a wafer to be irradiated with ultraviolet light having a wavelength in water of 134 nm. This technology is referred to as “ArF immersion exposure” or “ArF immersion lithography”.
Spectral line widths of KrF and ArF excimer laser apparatuses in natural oscillation are as wide as approximately 350 pm to 400 pm. This may cause chromatic aberration by using exposure lenses that are made of a material that transmits ultraviolet light such as KrF and ArF laser beams. The chromatic aberration thus causes a reduction in resolution. Accordingly, the spectral line width of the laser beam outputted from the gas laser apparatus needs to be narrowed to such an extent that the chromatic aberration can be ignored. To narrow the spectral line width, a laser resonator of a gas laser apparatus may be equipped with a line narrow module (LNM) having a line narrow element. The line narrow element may be an etalon, a grating, or the like. A laser apparatus whose spectral line width is narrowed is hereinafter referred to as “line narrowed laser apparatus”.
Patent Document 1: International Publication No. WO2015/068205 A
Patent Document 2: Japanese Patent Application Publication No. 2000-306813 A
Patent Document 3: U.S. Pat. No. 6,330,261 B